The MSDN documentation for implementing the policy has a pretty clear explanation:
Applications include a combination of business logic and cross-cutting issues, and the two are typically confused - which can make code harder to read and maintain. Each task or feature of the application is called a "problem." Concerns that implement the features of an object in an application, such as business logic, are major issues. Crosscutting problems are necessary tasks, functions or processes that are different objects - for example, registration, authorization, verification, and devices. The purpose of the policy implementation application block is to separate the main issues and the cross-cutting issues.
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